How can I change the directory used to cache BVH?
From the blender 2.62 release notes


  For renders where only the camera or materials are changing, while meshes stay fixed, it is now possible to skip the sometimes slow BVH step, by enabling BVH Cache in the Performance panel. This will store the BVH on disk for subsequent (animation) renders of the same scene. Note that if the scene does change in the animation, enabling this option will make rendering somewhat slower.


The best way to tell if your mesh is changing is to render a few frames, if you get more than one bvh file in the cache directory then the mesh is changing and cache bvh probably isn't helping. Some simple tests can show that an armature deforming a mesh is considered altering a mesh and creates a new cache file each frame, the same as an object moving in the scene, but a camera moving through a static scene can make use of one cache file.

5GB of cache files sounds extreme, of note here is that blender doesn't appear to clear this cache out by itself, you may want to delete everything in your cache folder.

As for where it is saved, you should be able to delete the cache folder and replace it with a shortcut - be sure to call the shortcut cache. I know this works on *nix systems but should also work on windows.